1. Field of the Invention
This invention relates to a mechanical device for surgical replacement of certain bones, joints, and associated ligaments in a diseased or otherwise damaged human wrist, and more particularly to a total wrist prosthesis.
2. Information Disclosure Statement Pursuant to 37 CFR .sctn.1.97
Total wrist arthroplasty involves the prosthetic replacement of the wrist by an artificial joint designed to simulate normal wrist motion. Prosthetic replacement of the wrist requires careful consideration of anatomic and mechanical factors, appreciation of the techniques of implant fixation, and recognition of soft tissue balance across the wrist. Current cemented wrist implant designs have an unacceptably high rate of failure (15% to 30%) with a 5-year progressively decreasing "survival rate". See for example Cooney W. P., Beckenbaugh, R. D., Linscheid, R. L.: Total Wrist Arthroplasty-Problems with Implant Failures. Clin. Orth. Rel. Research 187: 121-128, 1984 (a copy of which is sent herewith).
Drawing kinematics of the wrist are very complex as depicted by Youm et al. Youm, Y. McMurtry, R. Y., Flatt, A. E., and Gillespie, T. E.: Kinematics of the Wrist, 1. An Experimental Study of the Radial-Ulnar Deviation and Flexion--Extension. J.Bone Joint Surg. 60A: 423, 1978. These motions could not be simply reproduced and incorporated in the design of a joint prosthesis. Based upon experience with previous wrist arthroplasties, and several key anatomic and kinematic principles found in the literature, a successful compromise is achieved by the present invention. In studies of hea1thy individuals with normal wrists, the center of rotation of the wrist has been found to be located within the head of the capitate. For radial-ulnar deviation (R-U-D), the center of rotation lies toward the ulnar side of the axis of the third metacarpal and the axis of the distal radius. The Center of flexion-extension motion (F-E-M) of the wrist was known to be offset toward the palm (volarly) from the axis of the third metacarpal and the distal radius, and also located in the head of the capitate. Pronation and supination are rotation motions of the arm about a longitudinal axis. The wrist does not rotate significantly in relation to the radius. The wrist has therefore been described as a biaxial joint with two degrees of freedom, F-E-M and R-U-D.
Furthermore, the index finger and the long metacarpals together with the capitate and lesser multangular form a "fixed unit." Very little relative motion occurs between these bones during all natural positions of F-E-M or R-U-D. Also, the head of the capitate contains the dual centers of rotation for both F-E-M and R-U-D. The capitate-third metacarpal fixed unit is therefore the logical positioning choice for the metacarpal component of the wrist prosthesis. The ellipsoidal head design of the present invention is further based upon the anatomy of the scaphoid-lunate-triquetrum unit. The proximal aspects of these bones, along with the interosseous ligaments, are covered by cartilage to form a continuous surface that move as a unit in R-U-D. The more anatomic ellipsoidal shape articulating component along with the appropriately shaped radial component in this device better approximates the function of the normal wrist joint than previous prosthetic designs.
Many variations and types of wrist prostheses have appeared and representative examples of these variations and types are disclosed by the following group of patent references. Each reference pertains in one way or another to a wrist prosthesis, though some references are believed to be more relevant to the present invention than others.
______________________________________ Patent No. Patentee ______________________________________ 4,259,752 Taleisnik 4,180,871 Hamas 4,063,314 Loda 4,106,128 Greenwald, et al 3,909,853 Lennox 4,307,473 Weber 4,352,212 Greene, et al 4,229,840 Gristina 4,229,841 Youm, et al 4,178,640 Buechler, et al 4,164,793 Swanson ______________________________________
Taleisnik discloses a prosthetic wrist joint having an "ovoid" head on the metacarpal component. Taleisnik also discloses a prosthesis with a single metacarpal fixation stem and a single radial fixation stem, wherein the radial fixation stem in the radial component is offset relative to the receiving surface of the radial component so as to shift the center of rotation ulnarlly and volarly. Taleisnik, however, discloses only a semi-cylindrical receiving surface in the radial component and thus the receiving surface is not complementarily shaped relative to the bearing surface, as is the case in the present invention.
Hamas depicts a wrist prosthesis in which the concave member is carried by the metacarpal component with the bearing head being carried by the radial component. Hamas pertains to a wrist prosthesis which has the axes of radial-ulnar deviation and of flexion-extension motion located in specific relationships. The general displacement of these axes from the radial axis and the metacarpal axis are generally in the same direction as contemplated by the present invention.
The remaining patents are of less interest. The Loda, Greenwald and Lennox patents all show a ball and socket combination used in the wrist joint with the ball member in each instance being carried by the metacarpal component. The Loda patent claims a prosthesis which includes a primary stem which extends from the ball or "rotulla" as it is called in the patent, and a secondary stem which extends outwardly from the primary stem and then is bent into a parallel relationship with the primary stem. The Greenwald patent describes a prosthesis which includes a ball on the end of the metacarpal component and received within a cup-shaped member which is in turn received by the radial component. The Lennox patent describes a more narrow concept in which the metacarpal components are rotatable relative to a carpal or intermediate component which is in turn rotatable relative to the radial component. Each of the Weber and Greene patents shows a cylindrical head of the metacarpal component received within a cylindrical recess of the radial component. The Gristina and Youm patents both show dual pivoting members, as does the Weber patent previously cited. The Buechler and Swanson patents are of only general interest and not directly related to the present invention.
The invention disclosed in this application is basically a biaxial unit. The stem of the radial component is offset so that the center of R-U-D is shifted ulnarward to correspond with the anatomic center of rotation of the normal wrist. Similarly, the radial stem is offset so that the center of F-E-M lies volar to the mid-shaft of the radius. In contrast, the Hamas device has a spherical approximation of a biaxial anatomic structure, and therefore does not and cannot separate the centers of rotation for F-E-M and R-U-D. Other wrist prostheses do not correctly approximate either center of rotation. As was previously stated, the design of the present invention is a successful compromise of several key anatomic and kinematic principles.